Normal adult human tissues and cultured bronchial epithelial cells and fibroblasts exhibit 06-alkylguanine-DNA alkyltransferase activity in vitro by catalyzing the repair of the promutagenic alkylation lesion 06-methylguanine from DNA. Alkyltransferase activity varies in the different human tissues tested in the decreasing order of liver greater than colon greater than esophagus greater than peripheral lung greater than brain. Various human tissues exhibit 2- to 1-fold higher alkyltransferase activity than corresponding rat tissues. The present results show that different human tissues and cells have a several-fold higher capacity to repair 06-methylguanine in DNA than rat tissues and that the repair process occurs via a mechanism similar to that previously shown in other mammalian cells and E. coli. Formaldehyde inhibits repair of 06-methylguanine and potentiates the mutagenicity of an alkylating agent, N-methyl-N-nitrosourea, in normal human fibroblasts. Because formaldehyde alone also causes mutations in human cells, formaldehyde may cause genotoxicity by a dual mechanism of directly damaging DNA and also inhibiting repair of mutagenic and carcinogenic DNA lesions caused by other chemical and physical carcinogens. In some experimental studies, repeated exposure to alkylating agents has lead to an increase in 06-methylguanine-DNA alkyltransferase activity, i.e., an adaptive response. We have shown that human bronchial epithelial cells do not adapt and increase their DNA repair capability. This finding has important implications in carcinogenesis caused by low doses of N-nitrosamines. Membrane damage by carcinogens and tumor promoters can also lead to generation of lipid peroxidation aldehydes and indirect DNA damage. We have recently shown that these aldehydes cause extensive DNA damage and inhibit DNA repair in human bronchial epithelial cells.